The present invention is related to a scanning probe microscope, typically known as an atomic force microscope (AFM). More specifically, the present invention is related to a scanning probe microscope capable of rapidly attenuating resonant vibrations produced in a probe drive system.
In a scanning probe microscope such as AFM, a cantilever, on which tip portion a probe is mounted, is used as a probe in order to detect very fine tissue and very fine structures of a sample surface by utilizing mutual effects occurred between the sample surface and the probe. Since either attracting force or repulsive force is produced by such force reacted among atoms between the probe and the sample surface, the force is detected as distortion amounts of the cantilever while the probe is scanned on the sample surface in the X/Y directions along the main scanning direction and the sub-scanning direction. When a sample stage is moved in a fine mode along the Z-axis direction in such a manner that this distortion amount, namely the space between the sample surface and the probe becomes a constant, the very fine movement signal, or the detected distortion amount itself may express the shape of the sample surface.
As the scanning mechanism for scanning the probe along the main scanning direction and also the sub-scanning direction, and as the very fine moving mechanism for moving the probe along the Z direction, a mechanism combining a stepper motor and a differential screw, and a mechanism using a piezoelectric actuator such as PZT have been proposed. Various improvements in view of dimensions and cost are required for these proposed mechanisms. To this end, the present inventors have developed such a scanning probe microscope for scanning and moving the probe in a very fine mode while using a voice coil motor which has been applied in an acoustic sound speaker. This scanning probe microscope is disclosed in Japanese Unexamined Patent Application No. 8-25201 (1996).
In case that the voice coil motors are employed as the scanning means and the fine moving means, the supporting mechanism for supporting the probe is mechanically coupled via the coupling means to the movable members of the respective voice coil motors. At this time, if the frequencies of the main scanning operation and of the sub-scanning operation, and also the frequency of the Z-fine moving operation (hereinafter referred to as "drive frequencies" hereinafter) are made coincident with the natural (characteristic) frequency of the probe drive system involving the voice coil motors and the supporting mechanism, then the probe drive system is resonated when the probe is driven, so that the probe is largely vibrated, or oscillated. Accordingly, the natural frequency "f0" of the probe drive system is designed in such a manner that this natural frequency is not made coincident with the main scanning frequency "fx",the sub-scanning frequency "fy", and the Z-fine moving frequency "fz". Concretely speaking, it is desirable to design such that the natural frequency f0 of the probe drive system is 5 to 10 times higher than the drive frequencies.
However, as a result of experiments conducted by the present inventors, even when the drive frequencies of the probe are not made coincident with the natural frequency of the probe drive system, since vibrations occur which contain the higher frequency component than the original scanning frequency at the returning time during the main scanning operation and the sub-scanning operation, the probe drive system may be resonated and be greatly vibrated.
FIGS. 3A-3C comparatively show a relationship between the scanning signals entered into the voice coil motors and the actual scanning positions of the probe. More specifically, FIG. 3A indicates the main scanning signal (drive current IX) used to scan the probe along the main scanning direction, and FIG. 3B represents the actual position (ordinate) of the probe scanned in response to the main scanning signal Ix with respect to the main scanning direction.
The probe drive system is not resonated just after the scanning operation is commenced until the first returning operation, but the scanning position of the probe may correctly follow the main scanning signal Ix. However, when the returning operation of the probe is commenced, since the scanning direction is instantaneously changed, the vibrations containing a higher frequency component than the original scanning frequency may be produced. As a result, the resonant vibrations may occur in the drive system. Accordingly, after the probe is scanned from the returning position, the drive system is relatively largely vibrated, so that the scanning positions are brought into unstable conditions. Thereafter, the resonance is gradually attenuated and then the drive system will become stable. However, when the next returning operation is again carried out, the probe is again resonated in the similar manner, so that the scanning positions are brought into unstable conditions.
As previously explained, the resonance occurred in the probe drive system is not considered in the conventional scanning probe microscope, and this resonance is caused by the vibrations produced when the probe is scanned from the returning position. As a consequence, there are such problems that the probe cannot be scanned under stable condition, and therefore the microscopic images are observed under unclear conditions.
The present invention has been made to solve the above-described problems, and therefore, has an object to provide a scanning probe microscope capable of rapidly attenuating resonant vibrations occurred in a probe drive system during probe scanning operations.